This invention relates precise control of the frequency of a laser, and also has application to using a laser beam for the precise control of distance.
In the laser spectroscopy art, practitioners are always striving for improvements in sensitivity and accuracy of control over the frequency of lasers. One prior art technique employs frequency offset locking, which allows lasers to be scanned with high precision under rf phase-lock control [see J. L. Hall, J. Quant Electr., QE-4, 638 (1968)]. With this technique, accuracy is very high, being fixed by rf frequency synthesizer technology, but the scan window is narrow. Also, the technique employs an additional stabilized reference laser which serves as the reference value against which the frequency-offset scans are made, and the expense and trouble associated with the need for an additional laser detract from the simplicity and general applicability of this and other techniques that require two lasers. For this and other reasons, it has been recognized that an optical interferometer, which uses the beam from the laser to be stabilized, could be advantageously used as the frequency reference element. Since the light wavelength is so small relative to laboratory dimensions, interferometers operate ordinarily in an overtone mode. Thus, by changing the interferometric order by one or several units, one can obtain equivalent sharp resonances for laser frequency control, but now separated by many MHz, GHz, or TeraHz from the original lockpoint. At this point one has a comb of spectral reference lines, but in the act of giving up the second laser one also gives up our avenue to fine-scan control in the domain between these reference lines.
To provide the interpolation capability to augment these order-by-order precise comb steps of laser frequency, prior art systems have been developed for guiding a dye laser (or other tunable laser, for example a laser diode). One such technique is based on the use of an efficient electro-optic modulator to create an optical sideband frequency-displaced from the laser carrier by the frequency of the applied rf signal. Contemporary electro-optic materials and careful design allow bandwidths in the few GHZ range. The tunable sideband can be kept at the resonance frequency of the reference interferometer by a servo control system. In this way, changes of the rf frequency will be mapped into identical changes in the laser's carrier frequency. A system of this type then can achieve essentially rf accuracy of the scanning relative to the stable, sharp frequency reference interferometer. [See M. D. Rayman, C. G. Aminoff, and J. Hall, J. Opt. Soc. Am., Proceedings of 1985 Annual Meeting.] While this approach eliminates the need for a second tunable laser, it still requires a precision rf source which can be expensive. Furthermore, the tuning range is limited to a few GHz unless provision is made to change interference orders.
A further existing approach can be understood relative to the previously-described frequency sideband technique as follows. In the sideband technique one uses a broadband electrooptic modulator to produce an optical sideband frequency which can be scanned with precision relative to the laser carrier. The sharp Fabry-Perot resonant cavity does not need to be tuned--the interpolation between orders is provided by the scan capability of the rf modulator-produced sideband. An alternative interpolation scheme could be based on the softer, sinusoidal fringes produced by a Michelson interferometer. With this choice, the frequency control information will necessarily be less precise, but it can be arranged to have some frequency information throughout the range corresponding to one full order.
The underlying basis of one such interpolation scheme was suggested in the prior art by Juncar et al. [See P. Juncar and J. Pinard, Opt. Comm. 14, 438 (1975); and Rev. Sci. Instr. 53, 937 (1982). ] This approach, called a "Sigmameter", makes use of the fact that an interferometer produces a map between optical frequency changes and the change in phase of the associated interference pattern. With optical phase delay techniques (total internal Fresnel reflection) the Sigmameter obtains two photosignals representing sine and cosine phase quadratures of the interference signal. The Sigmameter obtains phase, and uses it to control the optical frequency. However, the technique has not been widely applied because the information about the interference phase (and hence about the optical frequency) is only available as a dc signal, and therefore risks contamination by dc problems such as changing fringe visibility, stray light, and dc drift.
It is among the objects of the present invention to improve upon prior art techniques of the type set forth, and to generally improve the art of controlling laser frequency and controlling distance.